1. Field of the Invention
The present invention relates to the field of the generation of signals under constraints and more specifically the production of discretized signals with imposed spectrum for example for producing noise generators or scramblers. The invention relates in particular to the field of acoustic noise generators.
2. Prior Art
The qualities that are required of scramblers and in particular acoustic scramblers are, with a controlled volume and weight, to transmit a noise of imposed power spectral density (p.s.d.) with the greatest possible energy autonomy.
It is known from the prior art to proceed by associating with a driver synthesizing the signal to be transmitted, a power amplifier that supplies the electrical excitation at the same time as it achieves the electrical impedance matching of the transducer which transforms the electrical signal into an electromagnetic, optical, acoustic or other signal depending on the medium in which the scrambling or noise generation is performed.
The class of the power amplifier used (A, AB, B, C, D, E, F, . . . ) depends on the constraints of linearity, efficiency, maximum frequency, resolution, and so on, imposed operationally.
The use of conventional means in some cases poses serious problems. The transducers have complex, or reactive, frequency-dependent impedances that require adapters to be implemented after amplification that are themselves reactive—inductive or capacitive—possibly associated with magnetic transformers. These elements normally have a footprint and a weight that render their use prohibitive. Similarly, in certain particular applications, producing high power linear amplifiers with a wide dynamic range and wide bandwidth is not always compatible with the efficiency, weight and volume conditions imposed by certain applications.
To meet the requirements of efficiency and miniaturization of the linear power amplifiers, it is possible to produce a high amplitude electrical signal by placing in series a certain number of DC voltage sources of reasonable sizes and delivering voltages of suitable values. Each source delivers, via an appropriate switching system, a voltage that is generally constant to the duly produced series circuit. This way, under the action of a specific control, variable in time, applied to the corresponding switch, each source is placed in series with other sources which are also selected and the voltage V produced by this source is added to the voltages produced by the other sources so as to produce the required level.
In a simple embodiment, it is possible, for example, to use a single voltage source whose outputs can be switched so as to generate a signal in the form of a two-level coded sequence, having the power needed to excite a transducer. The switching actions can, for example, by controlled according to a binary sequence of maximum length, repeated to produce a white noise. This solution is very satisfactory with respect to the energy but does not make it possible to take account of the spectral constraints that are more often than not imposed in the noise generators, especially when their function is to deceive.
One way of proceeding is to start from an analog signal, synthesized by a pilot having the required characteristics, and to select sources by discretizing the signal from the pilot by as many levels as are needed to calculate the source switchover indicators. The product signal obtained is applied to the transducer, it has the desired power but is discretized and does therefore have exactly the same p.s.d. as the DC signal. There are many limitations to this method. To avoid distorting the p.s.d. of the signal caused by the discretization there is a trend towards multiplying the number of levels, that is, increasing the number of the voltage sources and therefore the weight and footprint. This also leads to an increase in the frequency of the level transitions which lowers the efficiency.
Methods other than this direct discretization are possible. One solution consists in applying a known synthesis method, of the “conversion with a small number of quantization bits (up to 1)” type, such as “Sigma-Delta” modulation for example. This solution, not expanded on here, is, however, rarely used to generate a noise having a determined spectrum. In principle, this type of synthesis solves the dynamic range problem. However, it favors, by the spectral configuration of the quantization noise intrinsic to its oversampled operation, the displacement of the energy of the quantization noise to the upper parts of the spectrum. A major filtering of this noise outside of the useful band is normally necessary, which radically reduces the benefit of this approach. One way of limiting these effects then consists in strongly increasing the sampling frequency of the system, a means which, for technological hardware or cost reasons, is not always usable.
Another way of producing the synthesis of the control signal consists in using a synthesis method based on the known principle of pulse width modulation. In this method, the transition instants are defined by the amplitude of the signal. The generation of out-band noise is limited and the accuracy of the coding is satisfactory. On the other hand, this synthesis method involves having a variable clock and therefore remains a solution that is relatively difficult to implement and, moreover, costly. It is possible to synchronize the transitions, but it is essential to choose, for the clock, a frequency that is much higher than the Shannon frequency: problems similar to those described previously then arise again.